1. Field of the Invention
The present invention relates to a power unit for a saddle-seat vehicle, and more particularly, to power unit for a saddle-seat vehicle in which a cylinder block is inclined rightwardly or leftwardly, and an exhaust pipe is disposed in proximity to the inclined side wall of the cylinder block.
2. Description of the Background Art
A power unit for a saddle-seat vehicle has been disclosed, for example, in Japanese Patent Laid-Open No. Hei 7-195949. In a saddle-seat vehicle which often travels off road, an exhaust pipe is disposed along a side wall of a cylinder block to prevent the exhaust pipe from striking obstacles and irregularities in the ground. In the background art vehicle disclosed in the above-described document, an exhaust pipe is disposed on a left side wall of a cylinder block inclined in the leftward direction of a vehicle body. In a saddle-seat vehicle in which a cylinder block in inclined, for example, in the leftward direction of the vehicle body as described in the above document, the right side wall of the cylinder block has a large space in the vertical direction and more air circulates therepast, so that the right side wall exhibits superior cooling ability as compared with the left side wall which has a small space in the vertical direction and circulates less air.
Accordingly, in the case where an exhaust pipe is disposed along the left side which has poor cooling ability, the right side wall of the cylinder block which has high cooling ability and exerts a large effect on the cooling performance of the engine, is less susceptible to thermal effects from the exhaust pipe, to thereby improve the cooling performance of the entire engine.
The above-described background art vehicle, however, is disadvantageous in that since push rods of a valve mechanism are disposed in the right side wall of the cylinder block opposite to the exhaust pipe, the cooling effect produced by the right side wall of the cylinder block which has a large effect on the cooling performance of the engine, is obstructed by a space for enclosing the push rods, and the cooling performance of the entire engine is not fully achieved.